A regulated power supply typically includes a transformer and rectifier to first provide an unregulated output which is thereafter regulated by either a switching or a series linear regulator. In the power supply which includes a linear regulator, there is a minimum unregulated voltage which must be provided, typically two to three volts above the desired regulated output potential. Under normal load conditions (which would serve to deplete the storage capacitors in the power supply when not fully driven by the line), minor variations in line potential, and therefore unregulated power supply voltage is tolerated by merely increasing the unregulated voltage so that it remains above the minimum required by the regulators at all times, including the times of minor line voltage drop-out. However, to increase the power supply tolerance to line voltage drop-out, the magnitude of the nominal unregulated power supply voltage is also increased which requires the regulator to dissipate a greater portion of power and thus reduces power supply efficiency. Therefore, the requirements of high efficient power supplies and toleration of line drop-out heretofor been viewed as mutually exclusive.
Attempts to increase the efficiency of power supplies include preceding the unregulated power supply with a switching regulator; however, the resulting power supply requires a finite response time in order to respond and provide the required unregulated voltage for the subsequent linear regulator, which may exceed that tolerable by the linear regulator, resulting in a drop in the desired regulated output voltage. Moreover, the addition of the switching power supply has its own detrimental effect on overall power supply efficiency. A further limitation in this approach lies in the addition cost of and size of the semiconductor switching circuitry hardware.